Device-like metrology targets

ABSTRACT

Metrology targets, production processes and optical systems are provided, which enable metrology of device-like targets. Supplementary structure(s) may be introduced in the target to interact optically with the bottom layer and/or with the top layer of the target and target cells configurations enable deriving measurements of device-characteristic features. For example, supplementary structure(s) may be designed to yield Moiré patterns with one or both layers, and metrology parameters may be derived from these patterns. Device production processes were adapted to enable production of corresponding targets, which may be measured by standard or by provided modified optical systems, configured to enable phase measurements of the Moiré patterns.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/719,226 filed on Dec. 18, 2019, which is a continuationapplication of U.S. patent application Ser. No. 15/442,111 filed on Feb.24, 2017, which claims the benefit of U.S. Provisional PatentApplication No. 62/462,877 filed on Feb. 23, 2017, and of U.S.Provisional Patent Application No. 62/442,226 filed on Jan. 4, 2017,which are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION 1. Technical Field

The present invention relates to the field of metrology, and moreparticularly, to design, production and measurement of device-likemetrology targets.

2. Discussion of Related Art

As device production processes advance, metrology copes with smallerdevice details which limit significantly the available overlay budget.Hence new types of targets are required to enable reliable and accuratemetrology measurements of small device details.

SUMMARY OF THE INVENTION

The following is a simplified summary providing an initial understandingof the invention. The summary does not necessarily identify key elementsnor limit the scope of the invention, but merely serves as anintroduction to the following description.

One aspect of the present invention provides a metrology targetcomprising, between a bottom layer and a top layer each having acorresponding periodic structure, at least one supplementary structureconfigured to interact optically with at least one of the bottomperiodic structure and the top periodic structures.

These, additional, and/or other aspects and/or advantages of the presentinvention are set forth in the detailed description which follows;possibly inferable from the detailed description; and/or learnable bypractice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the invention and to showhow the same may be carried into effect, reference will now be made,purely by way of example, to the accompanying drawings in which likenumerals designate corresponding elements or sections throughout.

In the accompanying drawings:

FIGS. 1A-1D are high level schematic cross section illustrations ofmulti-layered target structures, according to some embodiments of theinvention.

FIGS. 2A and 2B are high level schematic illustrations of targets thatgenerate Moiré patterns, according to some embodiments of the invention.

FIGS. 3A-3H are high level schematic illustrations of target structuresand configurations, according to some embodiments of the invention.

FIGS. 4A-4D are high level schematic illustrations of productionprocedures, according to some embodiments of the invention.

FIGS. 5A-5D are high level schematic illustrations of optical systemswhich may be used to measure the targets, according to some embodimentsof the invention.

FIG. 6 is a high level schematic flowchart of a method, according tosome embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Prior to the detailed description being set forth, it may be helpful toset forth definitions of certain terms that will be used hereinafter.

The term “layer” as used in this application refers to any target layerproduced in the photolithography process such as a polysilicon layer, acontact layer, a resist etc. The term “structure” as used in thisapplication refers to any kind of designed or produced structure in atleast one layer of a metrology target. The term “periodic structure” asused in this application refers to any kind of designed or producedstructure in at least one layer which exhibits some periodicity. Theperiodicity is characterized by its pitch, namely its spatial frequency.Elements of the periodic structure may be segmented, i.e., comprisesegments that reduce the size of the minimal target feature. An examplefor a typical periodic structure is a grating. It is noted that periodicstructures may represent device elements, or may be device elements.

The term “metrology target” as used in this application refers to astructure that is used to derive measurements that are indicative ofproduction parameters such as overlay between layers, structuredimensions and any other accuracy merits. Metrology targets may bededicated structures which are produced for conducting metrologymeasurements thereupon, or may at least in part comprise actual deviceelements, that are used to derive measurements at certain productionsteps. The terms “bottom layer”, “previous layer”, “top layer and“current layer” as used in this application refer to a layer (bottom, orprevious) in the metrology target which is deeper and earlier producedthan another layer (top, or current) in the target structure. The term“supplementary structure” as used in this application refers to anystructure which is added to a design of a metrology target.Supplementary structure may be or comprise periodic structures but arenot necessarily periodic, and may be designed and produced in any targetlayer, including the top and bottom layers and above or below these,respectively. The term “intermediate layer” as used in this applicationrefers to a structure produced in lithography step at a layer that isdifferent from the bottom or top layers. The intermediate layer isillustrated in the following to be between the bottom and top layers.While the illustrations depict supplementary structures as for the sakeof simplicity, it is emphasized that the supplementary structure(s) maybe not intermediate and/or not periodic, as explained below. In certainembodiments, the supplementary structure(s) may be at the same layer asthe previous or current layers. It is noted that same numerals are usedto denote a layer and a respective structure in the layer, in order tosimplify the explanations.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

Before at least one embodiment of the invention is explained in detail,it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is applicable to other embodiments or of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting.

Metrology targets, production processes and optical systems areprovided, which enable metrology of device-like targets. Supplementarystructure(s) may be introduced in the target to interact optically withthe bottom layer and/or with the top layer of the target and targetcells configurations enable deriving measurements ofdevice-characteristic features. For example, supplementary structure(s)may be designed to yield Moiré patterns with one or both layers, andmetrology parameters may be derived from these patterns. Deviceproduction processes were adapted to enable production of correspondingtargets, which may be measured by standard or by provided modifiedoptical systems, configured to enable phase measurements of the Moirépatterns.

While it was suggested in the past to create Moiré patterns using thetop and the bottom gratings of overlay targets, the signal of prior artstructures was often insufficient for optical far-field measurementssince the evanescent modes of each grating in this approach decayexponentially and result in weak signals at most due to the largeoptical path between the gratings (in the order of hundreds of nm ormore).

Advantageously, the metrology targets enable overlay (OVL) and\orcritical dimension (CD) measurement of device patterns, as well assensitive pitch walk measurements, using intermediate lithographystep(s) that are part of the process flow. The targets utilize thestrong near field interaction between the previous layer andintermediate layer to yield detectable signals, and enable the previouslayer pattern to be much smaller than typical optical resolution limit.These targets thus improve the correlation to the device and the processcompatibility.

FIGS. 1A-1D are high level schematic cross section illustrations ofmulti-layered target structures 150, according to some embodiments ofthe invention. Targets 150 may comprise a bottom layer having a bottomperiodic structure 90, a top layer having a top periodic structure 80and supplementary structures 110, 130 configured to interact opticallywith at least one of bottom periodic structure 90 and top periodicstructure 80. Any number and types of layers 93 may be included intargets 150. Supplementary structures 110, 130 may be periodic, compriseperiodic elements and/or be non-periodic.

Supplementary structure(s) 110, 130 may be designed to improve theprocess robustness of the targets (CMP—chemical mechanicalplanarization). For example, structure(s) 110 and/or 130 may besegmented orthogonally to structures 80, 90 to minimize their effect onthe metrology measurements. In certain embodiments, structure(s) 110and/or 130 may be designed to improve metrology measurements by any of:(i) maximizing signal transmission from bottom layer 90, (ii) preventingsignal transmission from below layer 110, (iii) optimizing the bottomand top layers interference signal in SCOL targets, (iv) interactingoptically with any or both periodic structures 90, 80, e.g., bygenerating Moiré pattern(s) therewith, and specifically enablingmetrology measurement of small scale details like device elements, atleast in bottom layer 90, (v) interacting optically with any or bothperiodic structures 90, 80 to yield Moiré pattern(s) in two directions,enabling simultaneous measurements of overlay and CD (criticaldimension), and (vi) functioning as an apodizer to reduce finite targetsize effects. Some of these possibilities are exemplified in moredetails below.

FIG. 1A for example, illustrates the supplementary structure asintermediate layer 110 designed as a blocking layer or barrier layerthat removes disturbances from lower layers 93 (and specifically 90) tometrology measurements of target elements above barrier layer 110.Barrier layer 110 may enable re-using a given area for multiplemeasurements of different layer and different overlays withoutdisturbances from structures which were previously measured.

FIG. 1B, in another example, schematically illustrates supplementarystructure 130 which may interact optically with bottom periodicstructure 90 and/or with top periodic structure 80. In the illustratedexample, bottom layer has small scale details (e.g., tens of nm), e.g.,device or device-like features, while top layer has larger scale details(e.g., hundreds of nm), typical for providing optical metrologymeasurements. Supplementary structure 130 may be designed at a scalesimilar to bottom periodic structure 90 and to interact opticallytherewith. Simulations with various top layer CD and intermediate layerspace CD's have shown sensitivity of the overall metrology measurementsof targets 150 to structure details of bottom periodic structure 90,i.e., to details in the scale of several tens of nm. In the simulatedexample, the pitches of structures 90, 130 were 80 nm and 90 nmrespectively and the pitch of structure 80 was 720 nm.

FIGS. 1C and 1D schematically illustrate bottom layer periodic structure90 comprising multiply patterned elements 109 which are produced inmulti-patterning steps (i.e., elements produced by double, quadruple orgenerally multiple patterning), for example—finFET (“fin” field effecttransistor) elements, which may have an even smaller pitch of few tensof nm. FIG. 1C and FIG. 1D illustrate examples of multiply patternedelements 109 (as finFETs) produced by quadruple patterning andexhibiting a periodicity in groups of four elements 109. Using a cutprocess, fin elements 109 may be remove selectively and periodically,e.g., in FIG. 1C every fifth fin element is removed (the gap beingmarked 109A) while in FIG. 1D every fourth fin is removed 109A.Supplementary structure 130 may be configured to interact optically withbottom multiply patterned (e.g., finFET) periodic structure 90 andenable optical metrology measurements thereof. In certain embodiments,target 150 may comprise four cells, wherein in each of the cells adifferent element in each repeating group is removed 109A, to enablemeasuring the inaccuracy in the production of each aspect of thepatterning process separately. Simulations have shown that the resultingmeasurements are sensitive to the placement error of selected missingelements 109A, e.g., when respective Moiré patterns are generated byappropriately designed intermediate layer 130. In the simulated example,the CD of finFET elements 109 was 10 nm and their pitch was in the orderof 40 nm.

Hence, in certain embodiments, bottom layer 90 of targets 150 maycomprise a plurality of periodic multiply patterned elements 109,produced in multiple patterning steps, with which supplementarystructure 130 is configured to interact optically. Elements 109 maycomprise recurring sets of corresponding elements 109, and target 150may comprise multiple cells, each lacking a different one of theelements (109A) in the recurring set. Multiply patterned elements 109may be FinFET elements.

FIGS. 2A and 2B are high level schematic illustrations of targets 150that generate Moiré patterns, according to some embodiments of theinvention. FIGS. 2A and 2B present schematic cross section illustrationsof imaging targets 151 and SCOL targets 152 (e.g., first order SCOL)respectively, as examples for targets 150. It is explicitly noted, thatwhile in the illustrated examples, supplementary structure s 130 arephysically between top and bottom periodic structures 80, 90,supplementary structures 130 may as well be located below bottomperiodic structures 90, and even may be part of the lowest siliconlayer.

In certain embodiments, bottom and top periodic structures 90, 80respectively, have a same pitch (P) and supplementary structure 130 maybe a periodic structure having a pitch (P+ΔP) that is different from thesame pitch (P) to an extent that generates a detectable Moiré patternbetween periodic structures (90, 130 and 80, 130).

Corresponding Moiré patterns may be generated using supplementarystructure 130 (e.g., having a silicon grating) for measuring overlaybetween layers 80, 90. Targets 150 may overcome the difficulty thatintroducing supplementary structure 130 potentially involves new unknownparameter(s) like the overlay between supplementary structure 130 andother layers 93, including periodic structures 80, 90. Thus, whilesupplementary structure 130 may help in pitch reduction, it may requireadditional cells and additional measurements to determine its overlay.However, the proposed solution, in which supplementary periodicstructure 130 creates Moiré patterns with both bottom and top periodicstructures 90, 80 by having pitch P+ΔP which is slightly different(larger or smaller) from pitches P of bottom and top periodic structures90, 80.

In certain embodiments, target 150 may be configured as imaging target151 having at least a first cell 151A with top and supplementarystructures 80, 130 respectively, and a second cell 151B with bottom andsupplementary structures 90, 130 respectively. While pitches P and P+ΔPmay be unresolved, Moiré pitch P(P+ΔP)/ΔP may well be resolved bymetrology tools. Denoting the position of the resist grating (topperiodic structure 80) as UG (for “upper grating”); the position of theprocess layer grating (bottom periodic structure 80) as BG (for “bottomgrating”); and the position of supplementary structure 130 (which may bebelow bottom periodic structure 90) as IG (for “intermediate grating”,in a non-limiting manner)—two overlays may be defined, namely OVL1=UG-BG(which is the value the metrology intends to report) and OVL2=UG-IG(which is a byproduct of the target design). Considering a simplifiedmodel with only 0 and ±1 diffraction orders and normal illumination,noting that the most general case yields similar results, measuringfirst cell 151A provides a sum of zero and first diffraction orders withthe difference of topographic phases between zero and first diffractionorders being denoted as Ψ. First diffraction orders with effective pitchare the result of double scattering on intermediate (supplementary) andresist layers (130, 80 respectively, top layer 80 is also termed resistor coarse pitch layer), i.e., +1 coarse diffraction order=+1 resistdiffraction order and −1 intermediate diffraction order and vice versa,as expressed in Equation 1:

$\begin{matrix}{E\overset{\sim}{=}{{e^{{ikx}\;{\sin{(\theta_{0})}}}\left( {{A_{0}e^{i\;\Psi}} + {A_{1} \cdot e^{{i\;\frac{2\;\pi}{P}{({x - {UG}})}} - {i\;\frac{2\pi}{P + {\Delta\; P}}{({x - {IG}})}}}} + {A_{1} \cdot e^{{{- i}\;\frac{2\pi}{P}{({x - {UG}})}} + {i\;\frac{2\pi}{P + {\Delta\; P}}{({x - {IG}})}}}}} \right)} = {e^{{ikx}\;{\sin{(\theta_{0})}}}{\quad\left( {{A_{0}e^{i\;\Psi}} + {2A_{1}{\cos\left\lbrack {{\frac{2{\pi\Delta}\; P}{P\left( {P + {\Delta\; P}} \right)}x} - {\frac{2\pi}{P}{UG}} + {\frac{2\pi}{P + {\Delta\; P}}{IG}}} \right\rbrack}}} \right)}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$Substituting IG=UG-OVL2, the measured signal is a function of

${x - {UG} - {{OVL}\; 2\frac{P}{\Delta P}}},$i.e., may be denoted as

${F_{1}\left( {x - {UG} - {{OVL}\; 2\frac{P}{\Delta P}}} \right)}.$For second cell 151B, similar considerations yield Equation 2:

$\begin{matrix}{E = {{e^{{ikx}\;{\sin{(\theta_{0})}}}\left( {B_{0} + {B_{1} \cdot {\cos\left\lbrack {{{- \frac{2\pi}{P}}{BG}} + {\frac{2\pi}{P + {\Delta\; P}}{IG}} + {2\pi\; x\;\frac{\Delta\; P}{P\left( {P + {\Delta\; P}} \right)}}} \right\rbrack}}} \right)}=={e^{{ikx}\;{\sin{(\theta_{0})}}}\left( {B_{0} + {B_{1} \cdot {\cos\left\lbrack {{{- \frac{2\pi}{P}}{UG}} + {\frac{2\pi}{P}{OVL}\; 1} + {\frac{2\pi}{P + {\Delta\; P}}{IG}} + {2\pi\; x\;\frac{\Delta\; P}{P\left( {P + {\Delta\; P}} \right)}}} \right\rbrack}}} \right)}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Substituting IG=UG-OVL2 and BG=UG-OVL1, the measured signal is afunction of

${x - {UG} - {{OVL}\; 2\frac{P}{\Delta P}} + {OVL1{\left( {P + {\Delta P}} \right)/\Delta}P}},$i.e., may be denoted as

${F_{2}\left( {x - {UG} - {{OVL}\; 2\frac{P}{\Delta P}} + {OVL1{\left( {P + {\Delta P}} \right)/\Delta}P}} \right)}.$The difference between the two cell measurement functions depends onlyon OVL1 (and not on IG) and therefore overlay may be expressed as

${{{OVL} = {- \frac{\Delta P}{P + {\Delta P}}}} \cdot {Distance}},$with Distance being the difference in the OVL as measured using the twocells 151A, 151B.

In certain embodiments, target 150 may be configured as scatterometryoverlay (SCOL) target 152 comprising bottom, intermediate and top layers80, 130 and 90 respectively, in at least one cell. In this case, thefirst order signal in the pupil, for example, may be expressed byEquation 3:

$E = {e^{{ikx}\;{\sin{(\theta_{0})}}}\left( {{A_{1} \cdot e^{{i\frac{2\pi}{P}UG} - {i\frac{2\pi}{P + {\Delta P}}IG}}} + {B_{1} \cdot e^{{i\frac{2\pi}{P}BG} - {i\frac{2\pi}{P + {\Delta P}}IG}}}} \right)}$In this case the measured signal (which is the intensity) doesn't dependon position of intermediate grating 130 at all.

These derivations provides a large flexibility in positioningsupplementary structure(s) 130, while enabling the use of supplementarystructure(s) 130 in bridging small scale device features and the largerscale required for optical metrology measurements.

FIGS. 3A-3H are high level schematic illustrations of target structuresand configurations, according to some embodiments of the invention.FIGS. 3A-3H schematically illustrate targets 150A-H respectively andinclude both cross sectional illustrations as well as high level cellsorganization of targets 150.

Certain embodiments comprise metrology targets 150 comprising at leasttwo SCOL cells 152 having opposite designed offsets (+f₃, −f₃) betweentop periodic structure 80 and bottom periodic structure 90 and at leastone imaging cell 151 lacking top periodic structure 80 and having atleast three supplementary structure s 130, two of which having oppositeoffsets (+f₂, −f₂) and one of which having no offset (0) ofsupplementary structure 130 with respect to bottom periodic structure90.

Supplementary structure(s) 130 may be configured to yield Moiré patterns151E by optical interaction with corresponding bottom periodicstructures 90, which are measured with respect to corresponding topperiodic structures 80. The formation of Moiré patterns 151E isillustrated schematically in a top view 151D of structures 90, 130 andthe resulting optical interaction 151E between them.

FIG. 3A schematically exemplifies targets 150A having SCOL cells 152with layers 90, 130, 80 and imaging cells 151 with layers 90, 130,according to some embodiments of the invention. SCOL cells 152 may bemeasured to find the overlay between top periodic structure 80 (e.g., aresist layer) and the Moiré pattern resulting from the opticalinteractions of layers 90, 130, while imaging cell(s) 151 may be used todetermine the overlay between supplementary structure(s) 130 and bottomperiodic structure 90. Imaging cells 151 are exemplified in anon-limiting manner in an AIM (advanced imaging metrology) design havingthree pairs of cells: 151A and 151A′ with offset +f₂, having center153A; 151B and 151B′ with offset 0, having center 153B; and 151C and151C′ with offset −f₂, having center 153C. Imaging cells in aperpendicular direction may be added to the design. It is noted thatother layers 93 including the top layer may have structure, but suchwhich do not interfere optically with supplementary structures 90, 130.The overlay between supplementary structure 130 and periodic structure90 can be extracted from these three centers (for example, if153C-153B=153B-153A then OVL=0).

In certain embodiments, imaging cell(s) 151 may be measured prior toproduction of top layer 80, i.e., prior to production of top periodicstructure 80, e.g., after the etch of supplementary structure 130, whileSCOL cells 152 may be measured after production of top periodicstructure 80 for the overlay between structure 80 and Moiré pattern 151C(which is measured directly using imaging cells 151).

FIG. 3B schematically exemplifies targets 150B having imaging cells 151with layers 90, 130, 80, according to some embodiments of the invention.Certain embodiments comprise metrology targets 150 comprising at leastone imaging cell 151 having at least three supplementary structure s130, two of which having opposite offsets (+f₂, −f₂) and one of whichhaving no offset (0) of supplementary structure 130 with respect tobottom periodic structure 90. For example, imaging cells 151 may beorganized as illustrated in FIG. 3A.

It is noted that in certain embodiments, the OVL measurements between 80and 90 may be replaced by any arrangement which conveys phaseinformation (see e.g., FIGS. 5A-5D below) such as interfering twodiffraction orders (e.g., zeroth and first orders), e.g., by usingfinite elements along the optical axis such as a specified finite fieldstop a specified finite cell size etc.

FIG. 3C schematically exemplifies targets 150C having imaging cells151A, 151B relating to different Moiré patterns, according to someembodiments of the invention. Each of at least two imaging cells 151 mayhave at least three supplementary structures 130, two of which havingopposite offsets and one of which having no offset of supplementarystructure 130 with respect to bottom periodic structure 90, wherein inone of imaging cells 151, supplementary structure 130 may be configuredto form a detectable Moiré pattern with bottom periodic structure 90 andin another one of imaging cells 151, supplementary structure 130 may beconfigured to form a detectable Moiré pattern with top periodicstructure 80.

In certain embodiments, supplementary structure 130 can interfere withtop periodic structure 80 to form one Moiré pattern (e.g., having pitchP₂) as well as with bottom periodic structure 90 to form another Moirépattern (e.g., having pitch P₁), as illustrated schematically in FIG.3C. Different illumination wavelengths and/or apodizations and/orillumination angles may be selected to determine which of the modespropagates and accordingly which Moiré pitch is measured. Differentimaging cells 151A, 151B may be designed to enable measurement ofdifferent Moiré pitches P₁, P₂ in different cells 151A, 151B, forexample by measuring P₁ before production of layer 80 and/or usingbarrier layer 110 above bottom layer 90 before measuring P₂.Alternatively or complementarily, the measurement system may beconfigured to separate pitches P₁, P₂ at the image plane. Given andunknown overlay, designed offsets between layers 90, 130, 80 may bedesigned to enable at least three measurements, form which the unknownoverlay may be extracted.

FIGS. 3D and 3E schematically illustrates targets 150D, 150E beingarrays of imaging cells, according to some embodiments of the invention.In certain embodiments, metrology target 150 may comprise an array ofimaging cells, each cell having different combinations of designedoffsets ±f₂, ±f₃ between top, intermediate and bottom periodicstructures 80, 130, 90. Array 150D may comprise at least nine cells152A-J with all combinations of opposite and zero designed offsets, ±f₂,±f₃, 0, between the layers. Array 150E may comprise at least five cells152A-E, each cell having no offset or a single one of opposite offsets±f₂, ±f₃, 0, between bottom periodic structure 90 and one ofintermediate and top periodic structures 130, 80, respectively. Asbefore, different illumination wavelengths and/or apodizations and/orillumination angles may be selected to determine which of the modespropagates and accordingly which Moiré pitch is measured.

FIG. 3F schematically exemplifies targets 150F having SCOL cells 152 andan imaging cell 151 for measuring offsets with respect to finFETelements, according to some embodiments of the invention. Bottomperiodic structure 90 may be created using multiple lithography steps(for example: Self-Aligned Quadruple Patterning—SAQP and cut).

FIG. 3G schematically exemplifies targets 150G having two imaging cells151A, 151B, according to some embodiments of the invention. In certainembodiments, metrology target 150 may comprise at least two imagingcells 151A, 151B, each having at least three supplementary structure s130, two of which having opposite offsets (±f₂, ±f₃), and one of whichhaving no offset (0) of supplementary structure 130 with respect tobottom periodic structure 90. In one of imaging cells 151B the offset(±f₂) is between supplementary structure 130 and bottom periodicstructure 90 and in another one of imaging cells 151A the offset (±f₃)is between top periodic structure 80 and bottom periodic structure 90.

FIG. 3H schematically exemplifies targets 150H having two pairs of SCOLcells 152A, 152B, according to some embodiments of the invention.Metrology target 150 may comprise at least two SCOL (e.g., first orderSCOL) cell pairs 152A, 152B having opposite designed offsets, one pair152A having the designed offsets (±f₃) between top periodic structure 80and bottom periodic structure 90, and another pair 152B lacking topperiodic structure 80 and has the designed offsets (±f₂) betweensupplementary structure 130 and bottom periodic structure 90. In orderto measure the latter, a modified optical system may be used, forexample, modified optical systems disclosed below, e.g., in FIGS. 5A-5D.

FIGS. 4A-4D are high level schematic illustrations of productionprocedures 160, according to some embodiments of the invention. Deviceproduction procedures 160 (e.g., multiple patterning procedures) may beconfigured to enable production of any of targets 150 and in particularof supplementary structure 130, possibly without adding lithographysteps to the device manufacturing process. It is noted that whilesupplementary structure(s) 130 are illustrated in FIGS. 4A-4D asintermediate layers 130, supplementary structure(s) 130 may also beproduced at the same physical layer as bottom or top layers 90, 80(respectively) and/or below bottom layer 90 or above top layer 80. Incertain embodiments, several supplementary structures 130 may beproduced. It is further noted that supplementary structures 130 may butmust not be periodic.

FIG. 4A schematically illustrates the use of selective etch of devicepattern followed by deposition in to enable measurements of deviceedges, according to some embodiments of the invention. The additionaldeposition is needed in order to enable the measurements by deposingmaterial with different optical properties. Exemplary productionprocedures 160 comprises the following steps:

Starting (160A) with a pad with a periodic structure 161 (such as adevice grating, e.g., finFET elements with minimal pitch), lithographysteps (160B) may involve placing, exposing and developing resist 163Awith a pitch which is resolved by metrology. Then, etch steps 160Cfollow (possibly several process steps) in which resist pattern 163A ismoved to the previous layer pattern, including periodic structure 161.The last step of etch steps 160C may be a selective etch which removesonly one material from the bottom grating (periodic structure 161).Then, the etched volumes may be filled with material 164A which hassignificantly different optical properties that the etched outmaterial(s) in the bottom layer (160D) and the layers above the bottomgrating may be removed (160E), e.g., employing planarization. Remainingstructures 161A, 161B may be made of different materials. Followingdepositions of additional layers, imaging targets may be produced (160F)by placing, exposing and developing a resist pattern for the currentlayer (80) on top of a pad next to the pad processed in steps 160A-E,and/or scatterometry targets may be produced (160G) by placing, exposingand developing a resist pattern for the current layer (80) on top of thepad processed in steps 160A-E. It is noted that production procedures160 may be modified to yield various patterning options 165A, 165B,165C, depending on the parameters and directions of original periodicstructure 161 and of resist pattern 163A deposited in step 160B.

It is emphasized that production procedures 160 utilize the selectiveetch and deposition steps which are part of the device lithographyprocess—to create metrology targets, so that the metrology target edgesin one direction are defined by the device edges and generally toimprove the process compatibility of metrology targets 150.

FIG. 4B schematically illustrates the use of an additional hard mask163B in enabling measurements of the overlay between bottom periodicstructure 90 representing or actually being the device pattern, andcurrent layer pattern 163B. The measurement may be incorporated as partof the device production process, e.g., as part of Litho-Etch-Litho-Etch(LELE) production procedure 160 which is identical to the deviceproduction flow, according to some embodiments of the invention.Supplementary (intermediate) layer 130 with supplementary structures maybe produced above bottom layer 90 and have a pitch which is selected tointeract optically with bottom structure 90 and/or top structure 80(illustrated as mask 163B in FIG. 4B).

FIG. 4C schematically illustrates creating supplementary structure 130by production procedures 160, according to some embodiments of theinvention. Following the application of mask 163A (160B-C, comprisingdepositing the mask and removing material volumes which are notprotected thereby, respectively) to produce one type of material inintermediate layer 130 (160D), spaces between the elements are filledwith a second material 164A (160E) and finally top layer 80 isdeposited, possibly after measurements of supplementary structures 130and bottom layer 90, as explained above. It is noted that in standardmetrology targets there is no pattern in intermediate layer 130 (ofsecond material 164A).

FIG. 4D schematically illustrates creating a single layer, comprisingboth bottom periodic structure 90 and supplementary structure 130, andexhibiting a Moiré pattern upon optical measurement, according to someembodiments of the invention. The single layer may be produced usingmultiple lithography steps 160A-E, e.g., lithography and cut step(s) orLELE in production procedures 160. For example, the process may bedesigned to yield elements 161A of bottom periodic structure 90 andfilling elements 161B that define the positions for elements 164A ofsupplementary structure 130. In certain embodiments, the single layermay comprise supplementary structure(s) 130 made of a material 164A thatis different from the material of bottom periodic structure 90.

It is noted that even though the illustrations in FIGS. 4B and 4D maysuggest that structure 164A may violate some design rules (such asminimal tip to tip distance), this issue may be easily resolved bychoosing the proper pitches for structure 130 in FIG. 4B and forstructures 130 and 90 in FIG. 4D.

Advantageously, production procedures 160 overcomes inaccuraciesinvolved in current metrology targets which have target dimensionsdifferent from device dimensions, such as their different response toscanner aberrations and to other variables of the manufacturing processwhich may affect the overlay, the geometry, LER (line edge roughness)etc.

FIGS. 5A-5D are high level schematic illustrations of optical systems170 which may be used to measure targets 150, according to someembodiments of the invention. Optical systems 170 are designed to derivephase information, e.g., from targets 150, of Moiré patterns generatedby targets 150, and in certain cases in place of SCOL cells 152. It isnoted that in the far field overlays in Moiré patterns may result inphase differences rather than intensity differences between the 1st and−1st diffraction orders, and hence, phase information may be preferableto direct 1st order SCOL measurements. Illustrated optical systems 170demonstrate, in a non-limiting manner, implementations for interferingdiffraction orders in order to extract phase information they carry.

Optical systems 170 comprise an illumination source 61, illuminationfield stop and lenses 62, beam splitter 65 through which theillumination is delivered to objective 66 and onto target 150 on wafer60, and then back through optical elements such as lens 68A to detector69 such as a CCD (charge-coupled device).

FIGS. 5A and 5B illustrate the use of apodizer 64 and/or finite fieldstops 68 configured to diffract the orders so that specified orders,e.g., the zeroth and the first orders (+1 or −1) overlap and providephase information. Any type of apodizer 64 and field stop 68 may beused, e.g., diffractive or transmissive elements. Field stop 62 may beintroduced with corresponding optical elements to modify the length ofthe collection optical pathway.

Certain embodiments comprise metrology optical system 170 comprising atleast one of apodizer 64 and field stop 68, configured to interfere thezeroth order reflected diffraction signal with the first order reflecteddiffraction signal at a pupil plane in which detector 69 is located.

FIG. 5C illustrates exemplary optical system 170 for interfering +1diffraction order with −1 diffraction order by reflecting the capturedbeam using optical element 67D (e.g., comprising corresponding lenses)and superposing reflected pupil plane image 175B onto original pupilplane image 175A using beam splitters 67A, mirrors 67C and shutters 67B.

Shutters 67B are used to regulate beam intensities, to correct for beamattenuation during their manipulation (e.g., attenuation of thereflected beam). Specifically, an attenuation factor a can be measuredusing Equation 4, with E_(R) and E_(L) denoting the electric fieldresulting from each of the optical paths and (k_(x), k_(y)) denoting thepupil coordinates:E _(R)(k _(x) ,k _(y))=αE _(L)(−k _(x) ,k _(y))  Equation 4The signal resulting from the interference of +1 and −1 orders (whichhave positive and negative k values, respectively), which depends on theoverlay, may be expressed as in Equation 5:E(k _(x) ,k _(y))=E _(R)(k _(x) ,k _(y))=αE _(R)(−k _(x) ,k_(y))  Equation 5The measured intensity I(k_(x), k_(y)) depends of the overlay, asexpressed in Equation 6, with P₁, P₂ denoting the Moiré pitches and ε₁and ε₂ denoting the lateral position of the first and second gratings:

$\begin{matrix}{{I\left( {k_{x},k_{y}} \right)} \approx {{E_{R}}^{2}\left\lbrack {1 + a^{2} + {2a{\cos\left( {4{\pi\left( {\frac{ɛ_{1}}{P_{1}} - \frac{ɛ_{2}}{P_{2}}} \right)}} \right)}}} \right\rbrack}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

Equation 6 shows that the measured intensity depends on the OVL (forexample, by choosing ε=₁=0 and OVL=ε₂, without loss of generality.

Certain embodiments comprise metrology optical system 170 comprising anoptical assembly (e.g., beam splitters 67A, shutters 67B, mirrors 67Cand optical element 67D) configured to interfere a part of the reflectedsignal (175A) with a mirror image thereof (175B) to yield, at a pupilplane in which detector 69 is located, an interference of the +1 firstorder reflected diffraction signal with the −1 first order reflecteddiffraction signal. In FIG. 5C the location of 67C elements may bemodified mechanically to control the optical path. In addition phasemodulating element(s) may be added to control the phase of theillumination which is interfered with the signal.

FIG. 5D illustrates exemplary optical system 170 for interfering thepupil signal with a uniform field across the pupil to create overlaysensitivity. In the illustrated example, beam splitters 67A, mirror 67Cand attenuator 67E are used to introduce some of the illumination as theuniform field and adjust the intensity thereof, to interfere with thecollected signal. In certain embodiments, optical systems 170 mayinterfere the signal with an external uniform beam (for example, byreplacing mirror 67C with an additional illumination source).

Certain embodiments comprise metrology optical system 170 comprising anoptical assembly (e.g., beam splitters 67A, mirror 67C and attenuator67E) configured to interfere a part of the illuminated radiation withthe collected signal to derive phase information of the reflectedsignal.

In certain embodiments, optical system 170 may be configured to measurethe metrology signal in a field plane, in which the diffraction ordersinterfere, to derive the overlay from the phase information contained inthis interference. In certain embodiments, optical system 170 may beconfigured to provide ellipsometry measurements that contain therequired phase information.

FIG. 6 is a high level schematic flowchart of a method 200, according tosome embodiments of the invention. Method 200 may be at least partiallyimplemented by at least one computer processor, e.g., in a metrologymodule. Certain embodiments comprise computer program productscomprising a computer readable storage medium having computer readableprogram embodied therewith and configured to carry out of the relevantstages of method 200. Certain embodiments comprise target design filesof respective targets designed by embodiments of method 200. Certainembodiments comprise metrology targets produced by correspondingembodiments of method 200 and/or metrology measurements thereof.

Method 200 may comprise designing a metrology target with supplementarytarget structures, e.g., as one or more supplementary layer(s) (stage210) and/or performing metrology measurements on respective metrologytargets.

In certain embodiments, method 200 may comprise configuring thesupplementary layer as an optical separation layer (stage 212).

Method 200 may comprise incorporating a supplementary layer having asupplementary (possibly periodic or partially periodic) structure(s)between a bottom layer and a top layer of a metrology target (stage220), the supplementary structure configured to interact optically withat least one of a bottom periodic structure and a top periodic structurein the bottom and top layers, respectively (stage 230).

In certain embodiments, method 200 may comprise designing thesupplementary structure(s) to optically interact with bottom elements,produced in multi-patterning steps such as finFET elements (stage 232).Method may comprise removing, selectively and periodically, specifiedfinFET elements to yield a modified finFET layer (stage 235) andderiving metrology measurements from the interaction of thesupplementary structure with the modified finFET layer (stage 237).

Method 200 may comprise introducing designed offsets between the top,supplementary and bottom structures (stage 240) and using paireddesigned offsets with opposite signs (stage 245). Method 200 maycomprise designing a compound target comprising cells with differentoffsets between different layers (stage 250) and/or deriving metrologyparameters from measurements of the cells (stage 255). Method 200 maycomprise designing and/or using three layered SCOL cells and two layeredimaging cells, the latter lacking the top periodic structure (stage260). Method 200 may comprise combining two or more two-layered SCOLcells and/or two or more two-layered imaging cells, with different layerselections and different designed offsets (stage 270).

In certain embodiments, method 200 may further comprise configuring thesupplementary structure to yield a detectable Moiré pattern with atleast one of the top and bottom periodic structures (stage 280). Method200 may comprise configuring the supplementary structure to yielddetectable Moiré patterns with both the top and the bottom periodicstructures (stage 285), e.g., two Moiré patterns within three layeredcells, each layer comprising unresolved periodic structures (stage 285).The Moiré patterns may be measured with corresponding imaging cells(stage 287) and/or with corresponding SCOL cells. Method 200 maycomprise designing the supplementary structure to have a pitch slightlydifferent from a common pitch of the top and bottom structures, to yieldMoiré patterns of the same pitch with either layer (stage 290), e.g.,designing Moiré pattern generating cells as SCOL and/or imaging cells(stage 295).

Method 200 may further comprise producing metrology targets using a(possibly modified) device production flow (stage 300) and using deviceelements as part of the target. Method 200 may comprise utilizing thedevice production flow to introduce supplementary structure(s) to themetrology targets (stage 305). Method 200 may use process steps of thestandard device manufacturing process that are usually ignored in themetrology target design, resulting in the possibility to use withoutmodification the device production flow.

For example, method 200 may comprise producing the supplementary layerby one or more hard mask exposures (stage 307). Method 200 may comprisedepositing two or more substances to generate the supplementarystructure (stage 310). Method 200 may further comprise modifyingparameters of supplementary structure elements using recurring etchingand deposition (stage 315). In certain embodiments, the supplementarystructure may be configured to yield a detectable Moiré pattern with atleast one of the top and bottom periodic structures.

Method 200 may comprise deriving imaging and/or SCOL metrologymeasurements from the produced targets (stage 320). Method 200 mayfurther comprise resolving phase information from Moiré patternmeasurements of the target (stage 325), e.g., by interfering a SCOLfirst order signal from the target, by at least one of: manipulating theillumination, interfering the signal with a reference beam andinterfering first with minus first order signals (stage 330) or possiblywith the zeroth order signal.

In certain embodiments, method 200 comprises depositing an opticalseparation layer configured to block optical interaction with structuresbelow the deposited layer, and producing a metrology target above theoptical separation layer.

In the above description, an embodiment is an example or implementationof the invention. The various appearances of “one embodiment”, “anembodiment”, “certain embodiments” or “some embodiments” do notnecessarily all refer to the same embodiments.

Although various features of the invention may be described in thecontext of a single embodiment, the features may also be providedseparately or in any suitable combination. Conversely, although theinvention may be described herein in the context of separate embodimentsfor clarity, the invention may also be implemented in a singleembodiment.

Certain embodiments of the invention may include features from differentembodiments disclosed above, and certain embodiments may incorporateelements from other embodiments disclosed above. The disclosure ofelements of the invention in the context of a specific embodiment is notto be taken as limiting their use in the specific embodiment alone.

Furthermore, it is to be understood that the invention can be carriedout or practiced in various ways and that the invention can beimplemented in certain embodiments other than the ones outlined in thedescription above.

The invention is not limited to those diagrams or to the correspondingdescriptions. For example, flow need not move through each illustratedbox or state, or in exactly the same order as illustrated and described.

Meanings of technical and scientific terms used herein are to becommonly understood as by one of ordinary skill in the art to which theinvention belongs, unless otherwise defined.

While the invention has been described with respect to a limited numberof embodiments, these should not be construed as limitations on thescope of the invention, but rather as exemplifications of some of thepreferred embodiments. Other possible variations, modifications, andapplications are also within the scope of the invention. Accordingly,the scope of the invention should not be limited by what has thus farbeen described, but by the appended claims and their legal equivalents.

What is claimed is:
 1. A metrology target comprising: a top layer havinga top periodic structure; a bottom layer having a bottom periodicstructure, wherein the bottom periodic structure comprises a pluralityof periodic multiply patterned elements; and at least one supplementarystructure configured to interact optically with at least one of thebottom periodic structure or the top periodic structure such that aMoiré pattern is generated using the supplementary structure, whereinthe at least one supplementary structure is fabricated of a differentmaterial than the bottom layer, and wherein the supplementary structureis positioned outside the top layer and the bottom layer such that thesupplementary structure is above the top layer or below the bottomlayer.
 2. The metrology target of claim 1, wherein the at least onesupplementary structure is configured to interact optically with thebottom periodic structure.
 3. The metrology target of claim 2, whereinthe multiply patterned elements comprise recurring sets of correspondingelements, and wherein the bottom periodic structure comprises multiplecells, each of the cells lacking one of the multiply patterned elementsin the recurring set.
 4. The metrology target of claim 1, wherein thebottom and top periodic structures have a same pitch and wherein the atleast one supplementary structure has a pitch that is different from thesame pitch to an extent that generates a detectable Moiré patternbetween the periodic structures.
 5. The metrology target of claim 1,comprising an array of imaging cells, each cell having differentcombinations of designed offsets between the at least one supplementarystructure and the top and bottom periodic structures.
 6. The metrologytarget of claim 1, wherein each of the periodic multiply patternedelements has a width that is narrower than a width of the supplementarystructure.
 7. The metrology target of claim 1, wherein the bottomperiodic structure is made of finFET elements.
 8. The metrology targetof claim 1, wherein the supplementary structure is above the top layer.9. The metrology target of claim 1, wherein the supplementary structureis below the bottom layer.
 10. A method comprising: providing ametrology target including: a top layer having a top periodic structure;a bottom layer having a bottom periodic structure, wherein the bottomperiodic structure comprises a plurality of periodic multiply patternedelements; and at least one supplementary structure configured tointeract optically with at least one of the bottom periodic structure orthe top periodic structure such that a Moiré pattern is generated usingthe supplementary structure, wherein the at least one supplementarystructure is fabricated of a different material than the bottom layer,and wherein the supplementary structure is positioned outside the toplayer and the bottom layer such that the supplementary structure isabove the top layer or below the bottom layer; and performing metrologymeasurements on the metrology target.
 11. The method of claim 10,wherein the supplementary structure interacts optically with multiplypatterned elements in the bottom periodic structure.
 12. The method ofclaim 11, further comprising removing, selectively and periodically,some of the multiply patterned elements to yield a modified multiplypatterned layer and deriving metrology measurements from the interactionof the at least one supplementary structure with the modified multiplypatterned layer.
 13. The method of claim 10, further comprisingconfiguring the at least one supplementary structure to yield adetectable Moiré pattern with at least one of the top periodic structureor bottom periodic structure.
 14. The method of claim 13, furthercomprising configuring the at least one supplementary structure to yielddetectable Moiré patterns with both the top and the bottom periodicstructures.
 15. The method of claim 10, further comprising producing theat least one supplementary structure within a device production processby one or more hard mask exposures.
 16. The method of claim 10, furthercomprising depositing two or more substances to generate the at leastone supplementary structure.
 17. The method of claim 16, furthercomprising modifying parameters of supplementary structure elementsusing recurring etching and deposition.
 18. The method of claim 17,wherein the at least one supplementary structure is configured to yielda detectable Moiré pattern with at least one of the top periodicstructure or bottom periodic structure, the method further comprisingresolving phase information from Moiré pattern measurements of thetarget.
 19. The method of claim 10, wherein the supplementary structureis above the top layer.
 20. The method of claim 10, wherein thesupplementary structure is below the bottom layer.